Most of the APIs require data to be gathered, analysed and updated on a daily or weekly basis (e.g. BIN lookup, IP Info, Geocode, IP blocklist, User-agent Info, Phone validate). Some need to connect directly to other networks (e.g. Phone verify, SMS verify, HLR lookup, Host Reputation, Currency Convert). These would be either impossible or quite difficult to implement in a library / local only environment. Others can just be a pain to implement (depending on your setup). Additionally, you gain the advantage of automatically inheriting updates to our algorithms and data sources.
Ah, I didn't realize that these were updated, I thought it just used a regex to verify the email, phone number, filter bad words, etc.
A lot of these seem useful and having APIs that need to be updated is a good VP, however for things like HTML to PDF, there are libraries that already do this.
People will likely use the HTML -> PDF API, but in terms of the new APIs you are developing, I think you will attract more customers with things that cannot be implemented simply with a library and instead need to be managed by you.